In many industrial processes, there are encountered hostile substances that need to be transported through flow conduits or stored in containers. Such substances can exhibit corrosive and/or erosive properties when interacting with their environment. There is therefore a need for systems and methods that over a duration of time are operable to monitor an impact of such corrosion and/or erosion in order to ensure safe operation of associated conduits or containers.
During production of oil and natural gas, such hostile substances consist of oil, water, gas, in many cases different amounts of sand, and also different chemical additives. Pressures and temperatures of the hostile substances are normally high and security for safe operation over a period of years is imperative. Therefore, there is a need for an accurate and robust measurement system that is operable to monitor parameters as a thickness of a wall of a conduit, since this thickness will certainly change over time. It is not sufficient to measure this thickness at a single location of a pipe or a conduit. Larger portions of the pipe or conduit must be monitored as well, and for this purpose the present invention has shown its effectiveness.
A thickness assessment procedure pertinent to the present invention comprises a comprehensive analysis of phase and group velocity dispersion characteristics of appropriate propagation modes of an acoustic signal utilized to implement the procedure. A choice of modes for the analysis constitutes an important issue when implementing the procedure, as not all modes are equally sensitive to variations in wall thickness.
However, a technical problem is that concomitant complications arising owing to mode overlapping and distortion have to be tackled and overcome for the procedure. Long term thickness monitoring necessarily has to face a fact that local thickness variations potentially comprise a significant percentage of a corresponding average wall thickness; most guided wave modes do not display a sufficient degree of robustness required to smoothly integrate these variations into a quantitative thickness assessment.
Other systems are known which seek to address a similar technical problem to that of the present invention. A first known method and system typically utilizes pulsed acoustic signals over larger distances of greater than 2 meters, and analyzes reflected echoes from such pulsed acoustic signals in order to determine a presence of material damage. Moreover, the reflected signal echoes are susceptible to providing an indication of a location and an approximate spatial extent of the material damage, but are not able to provide quantitative information about an average wall thickness reduction.
A second method and system in contemporary use is capable of performing spot measurements at a position whereat transducers are located on a pipe. In the second method and system, pure longitudinal transverse resonances of zero group velocity (ZGV) are readily exploited for performing local thickness measurements implemented in a pulse-echo mode of operation. Averaging the thickness measurements at several different locations whereat transducers are mounted on the pipe are used to obtain valuable information on the nominal average thickness of the pipe if the longitudinal velocity is known, or to estimate the longitudinal velocity from initial measurements on a calibration pipe of known wall thickness. This second method is only able to provide a very local estimate of pipe wall thickness, but is not able to give necessary information about an average wall thickness over a larger part of the pipe. If the wall thickness were changing more or less uniformly over a distance between the transducers, one could just average thicknesses measured at the different transducer locations. However, long-range inspection necessarily has to face a problem that an average of the two thicknesses measured at ends of a given inspection path is not representative to a true average between those two ends.
A third method and system involves utilizing a so-called ‘Low Group Velocity’ (LGV) measurements between one or more pairs of acoustic transducers, and analyzing a family of longitudinal and shear waves that travel at such lower regime group velocities along a path for obtaining an estimate of general wall thickness condition therealong. These low group velocity measurements are extremely sensitive to uneven variations in wall thickness along the aforesaid path of propagation; the measurements can be exploited to flag paths where uneven variations are detected.
Theoretical analyses, as well as practical applications, have shown that an LGV-based assessment breaks down when there are local thickness variations in an order of 10% of an absolute wall thickness of a pipe section, when these defects occupy a reasonable percentage of a line-of-sight between transducers involved for implementing the assessment. The LGV method is thus capable of providing an overall assessment of the ‘well being’ of the pipe section. Partly due to the curvature of the pipe section and partly to a divergence of an ultrasonic beam produced by the finite-aperture transducers, some ultrasonic energy will also be radiated sideways and will consequently be lost from an otherwise perfect standing wave generated. This sideways radiation represents a principal damping of transverse resonance and also is the source of propagating Lamb modes. Among these propagating Lamb modes are modes of particular interest that allow a natural extension of the aforesaid ZGV method to a pitch-catch mode of operation. LGV modes represent both shear and longitudinal bulk waves bouncing back and forth between surfaces of a plate at close to normal incidence; in consequence, the resulting transverse resonance frequencies include two families of dominantly shear and longitudinal vibration, respectively.
The LGV method is highly sensitive to wall thickness variations over a propagation path between transducers involved, wherein such sensitivity badly limits a feasibility of this otherwise very simple LGV method for performing long-range inspection. The LGV technique exhibits a problem, like other techniques that are very sensitive to wall thickness variations, that it is not able to average at all the wall thickness between a transmitter and a receiver employed to implement the method because of a very non-linear relationship between measured propagation parameters and the sought wall thickness. The LGV method produces sharp transmission peaks in its corresponding transmitted spectrum, which can be associated with shear and longitudinal transverse resonances. For example, a 10 mm thick pipe produces, among others, a distinct transmission peak at a frequency of ≈600 kHz that gradually increase to a frequency of ≈680 kHz in response to the wall thickness reducing to 9 mm. However, if there is a step down from a wall thickness of 10 mm to a thinner wall thickness of 9 mm, a thicker part of the pipe transmits therethrough ultrasonic energy at frequencies that are not transmitted through the thinner part, therefore the transmission peak does not shift to a new position between frequencies of 600 and 680 kHz, but simply disappears. Such disappearance occurs irrespective of whether such a step change is gradual rather than localized; a tapered section from 10 mm at one end to 9 mm at the other of a section of pipe would not transmit ultrasonic energy therethrough pertaining to a simple LGV method.